The human T-cell leukemia/lymphoma virus, HTLV-I, is thought to be directly associated with adult T-cell leukemia (ATL) and indirectly associated with other malignancies. More recently this virus has been shown to be associated with the neurologic disease, tropic spastic paraparesis (TSP). Studies are underway to examine the mechanism whereby the same virus appears to contribute to different types of diseases. Peripheral blood lymphocytes from HTLV-I seropositive individuals with the neurologic disease have been shown to undergo spontaneous prolifera- tion when placed in culture. We have undertaken experiments to assess the nature of the lymphocyte population that is proliferating and the mechanism whereby this response is induced. Lymphocytes from HTLV-I-infected and noninfected individuals were tested prior to and after 6 days of culture for cell-surface markers that define activated lymphocyte subpopulations based on 3H-thymidine incorporation. Virus production was measured by p24 antigen capture assays. CD4+ lymphocytes appear activated, expressing the activation markers, HLA-DR and CD25, as 3H-thymidine incorporation increased and virus production was observed. Addition of autologous sera to the cultures increased the level of activation severalfold suggesting a mechanism for cell activation. Using antisera that identifies virus protein expression on the cell surface, subpopulations of cells producing virus were separated by flow cytometry and RNA and DNA was prepared from these cells. Diseases that might be associated with HTLV-II infection (a retrovirus related to HTLV-I) are unknown. A patient infected with HTLV-II was diagnosed as having large granulocytic leukemia by cell surface marker studies. The malignant cells from the peripheral blood were isolated by flow cytometry and DNA was prepared for viral integration studies.